In order to achieve good speed control of DC motors without resorting to using tacho-generators, encoders or resolvers, load compensation techniques can be used. This technique involves measuring the motor current, multiplying the current reading by a resistance value and adding this value to the demand signal, thereby compensating for the motor resistance induced drop of speed with increasing load.
Unfortunately for brushed DC motors, the resistance value is not constant and tends to increase as the current falls by as much as 3:1.
PCT patent application number PCT/NZ2011/000191 by the present applicant describes a technique of profiling the current dependant resistance term and using it to compensate the motor. It is advantageous to periodically update the profile because it can change with motor temperature or motor usage. In order to measure motor resistance to develop the profile, driving power to the motor must be turned off for a short period so that a no-load back EMF potential is allowed to establish at the motor terminals. This off-pulse, particularly at high loading, can cause a clicking noise and a torque pulse. In cases where the drive system is for power wheelchairs or mobility scooters, the click and pulse could be objectionable for users.
The clicking noise derives from the changes of motor iron dimensions due to changes of magnetisation. This effect is termed magneto restriction. Its effect is dependent on the construction of the motor, the type of iron used and the change of current in the motor.
Motor torque is proportional to the current flowing so torque pulse effects will be dependent on current flow, system inertia, speed and vehicle suspension design.
These effects cannot be eliminated, only reduced.
An object of the present invention is to provide a system and/or method that reduces the effects of the off-pulse during back EMF sampling, or to at least provide the public with a useful choice.
The present invention aims to overcome, or at least alleviate, some or all of the afore-mentioned problems.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.
The background discussion (including any potential prior art) is not to be taken as an admission of the common general knowledge in the art in any country. Any references discussed state the assertions of the author of those references and not the assertions of the applicant of this application. As such, the applicant reserves the right to challenge the accuracy and relevance of the references discussed.